The present invention relates to structures that can be used for circuits that process optical signals and for other circuits. (The terms “optical” and “light” as used herein denote electromagnetic radiation of any spectrum, not limited to visible light; the terms “optical fiber” or just “fiber” denote an optical fiber cable.) The present invention also relates to etching of monocrystalline silicon substrates to provide suitable angles that can be used for optical and non-optical purposes.
Fiber optics is increasingly used to transmit information to and from electrical circuits. Energy conversion between optical fiber and electrical circuitry is performed by opto-electrical transducers. Miniature packages have been created which combine the transducers, the optical fiber, and electrical circuitry to achieve high speed and low power losses. One example is described in Hsu-Liang Hsiao et al., “Compact and passive-alignment 4-channel x 2.5-Gbps optical interconnect modules based on silicon optical benches with 45° micro-reflectors”, OPTICS EXPRESS, 21 Dec. 2009, Vol. 17, No. 26, pages 24250-24260, illustrated in FIGS. 1-3.
FIG. 1 shows optical fibers 104 (104.1 and 104.2) used to interconnect integrated circuits (chips) 110.1, 110.2 mounted on respective printed circuit boards (PCBs) 114.1, 114.2. Chip 110.1, fiber 104.1, and PCB 114.1 are part of a signal transmitting module 116.1. Chip 1101.2, fiber 104.2, and PCB 114.2 are part of a signal receiving module 116.2. Electrical signals from chip 110.1 are provided to an opto-electronic transducer 120.1 for conversion to light. Transducer 120.1 is an integrated circuit (IC or “chip”) containing a semiconductor laser (vertical-cavity surface emitting laser, “VCSEL”). Transducer 120.1 is mounted on a silicon interposer (silicon optical bench, or SiOB) 124.1 made using a silicon substrate 130.1. Conductive lines 134.1 transmit electrical signals from chip 110.1 to transducer 120.1. In response, the transducer produces optical signals in a vertical light beam 140.1. Light beam 140.1 is reflected by a mirror 144.1 formed of a gold layer deposited on the silicon interposer's surface inclined at 45° to the horizontal. The reflected beam from mirror 144.1 enters the optical fiber 104.1.
Fiber 104.1 is connected to a fiber 104.2 of module 116.2 by a connector 150. Module 116.2 is similar to module 116.1. The optical signals are emitted from fiber 104.2 in a horizontal beam 140.2, which is reflected by a 45° mirror 144.2 to travel vertically to a transducer 120.2. The mirror is part of a silicon interposer 124.2 made using silicon substrate 130.2. Transducer 120.2 is mounted on interposer 124.2. Transducer 120.2 is a photodetector integrated circuit which converts the optical signals into electric signals provided, via conductive lines 134.2, to chip 110.2. Interposer 124.2 and chip 110.2 are mounted on PCB 114.2.
FIGS. 2 and 3 illustrate a module 116 which can be 116.1 or 116.2. FIG. 2 is a top view, and FIG. 3 shows a cross section by a plane transversal to fibers 104. Each module 116.1, 116.2 has four fibers 104 (i.e. 104.1 or 104.2); transducer 120.1 has four lasers emitting four respective beams 140.1 entering four respective fibers 104.1; transducer 120.2 has four photodetectors which receive four respective beams 140.2 passing through four respective fibers 104.2. As shown in FIG. 2, in each module, monocrystalline silicon substrate 130 having (100)-orientation supports all the four fibers 104. The fibers are mounted in V-grooves 310 formed by a wet etch of substrate 130. The etch also forms the silicon surface underlying the mirror 144. The V-grooves have 45°-sloped sidewalls. The 45° angle is produced by an anisotropic wet etch of silicon substrate 130 which is a monocrystalline silicon wafer of (100)-orientation. The sloped sidewalls are (110) crystal planes. The etchant is a solution of KOH (potassium hydroxide) and isopropyl alcohol (IPA) chosen to suppress the etching rate of {110} planes to a level below the etching rate of {111} planes. The 45° angle so produced is highly precise, which helps in precise positioning of fibers 104 because the fibers do not reach the groove bottom and the fiber position is therefore determined by the angle of the grooves' sidewalls (45°) and the groove's width at the top.